Fat1 is large transmembrane protein of the cadherin super-family that localizes to the podocyte slit diaphragm. Deletion of fat1 in experimental mice results in abnormal development of the slit diaphragm, massive proteinuria, and death shortly after birth with nephrotic syndrome, a phenotype similar to mice deleted for nphs1 (Nephrin). This proposal includes background and preliminary data showing that Fat1 is required for the fundamental process of lateral polarity in epithelial cells, includes preliminary data that Fat1 interacts with conventional kinesin, and hypothesizes that the interaction between Fat1 and kinesin is critical for lateral polarity. I propose a model to explain why Fat1 -dependent lateral polarity is fundamentally important for podocyte development and function, which I will test using cultured podocytes and transgenic mice. This proposal also includes background on the regulation of actin dynamics by Fat1, which may occur through interaction with the actin regulatory protein VASP. During the course of this research, I will use techniques of protein biochemistry and cell biology, and I will receive training in the use of advanced microscopy and mouse transgenics. Fat1 research has applications to general cell biology, including cell polarity, intracellular protein transport, and protein-protein interactions. Fat1 research also directly applies to the path biology of glomerular disease due to its localization at the slit diaphragm and the severe phenotype of experimentalFat1 knockout mice. PUBLIC HEALTH RELEVANCE: With governmental and professional outreach, the public is becoming increasingly aware that glomerular disease is a major cause of morbidity, mortality, and public health expenditure, accounting for over 55% of patients starting hemodialysis in the United States. Our treatments for patients with glomerular disease to slow or prevent progression of disease are not sufficiently effective, and do not precisely target the causes of glomerular disease. This proposal seeks to extend our understanding of glomerular disease through the study of slit diaphragm protein Fat1. My goal is to understand the biology ofFat1 in a simplified cell culture model system, and then to extend these studies to the role of Fat1 in the glomerulus. This proposal is relevant to the goals of the NIDDK to promote research on basic kidney biology, including applications within Cell and Molecular Biology of the Kidney (protein trafficking and cell polarity) and within the Pathobiology of Kidney Disease (normal biology of glomerular cells, and proteinuria).